The present invention relates to a process for purifying a gas mixture comprising dinitrogen monoxide, at least comprising the at least partial condensation of a gas mixture G-I comprising dinitrogen monoxide to obtain a liquid composition C-1 comprising dinitrogen monoxide, and the contacting of the composition C-1 with a gas mixture M-1 to obtain a composition C-2 and a gas mixture M-2.
The prior art discloses various preparation processes and purification processes for dinitrogen monoxide. It is likewise known that dinitrogen monoxide can be used, for example, as an oxidizing agent for olefins.
For instance, WO 98/25698 discloses a process for preparing dinitrogen monoxide by catalytic partial oxidation of NH3 with oxygen. According to WO 98/25698, a catalyst composed of manganese oxide, bismuth oxide and aluminum oxide is used, which leads to dinitrogen monoxide with high selectivity. A similar catalyst system is also described in detail in a scientific study (Noskov et al., Chem. Eng. J. 91 (2003) 235-242). U.S. Pat. No. 5,849,257 likewise discloses a process for preparing dinitrogen monoxide by oxidation of ammonia. The oxidation takes place in the presence of a copper-manganese oxide catalyst.
In the process disclosed in WO 00/01654, dinitrogen monoxide is prepared by reducing a gas stream comprising NOx and ammonia.
The oxidation of an olefinic compound to an aldehyde or a ketone by means of dinitrogen monoxide is described, for example, in GB 649,680 or the equivalent U.S. Pat. No. 2,636,898. Both documents quite generally disclose that the oxidation can in principle be effected in the presence of a suitable oxidation catalyst.
The more recent scientific articles of G. I. Panov et al., “Non-Catalytic Liquid Phase Oxidation of Alkenes with Nitrous Oxide. 1. Oxidation of Cyclohexene to Cyclohexanone”, React. Kinet. Catal. Lett. Vol. 76, No. 2 (2002) p. 401-405, and K. A. Dubkov et al., “Non-Catalytic Liquid Phase Oxidation of Alkenes with Nitrous Oxide. 2. Oxidation of Cyclopentene to Cyclopentanone”, React. Kinet. Catal. Lett. Vol. 77, No. 1 (2002) p. 197-205 likewise describe oxidations of olefinic compounds with dinitrogen monoxide. A scientific article “Liquid Phase Oxidation of Alkenes with Nitrous Oxide to Carbonyl Compounds” by E. V. Starokon et al. in Adv. Synth. Catal. 2004, 346, 268-274 also includes a mechanistic study of the oxidation of alkenes with dinitrogen monoxide in the liquid phase.
The synthesis of carbonyl compounds from alkenes with dinitrogen monoxide is also described in various international patent applications. For instance, WO 03/078370 discloses a process for preparing carbonyl compounds from aliphatic alkenes with dinitrogen monoxide. The reaction is carried out at temperatures in the range from 20 to 350° C. and pressures of from 0.01 to 100 atm. WO 03/078374 discloses a corresponding process for preparing cyclohexanone. According to WO 03/078372, cyclic ketones having from 4 to 5 carbon atoms are prepared. According to WO 03/078375, cyclic ketones are prepared under these process conditions from cyclic alkenes having from 7 to 20 carbon atoms. WO 03/078371 discloses a process for preparing substituted ketones from substituted alkenes. WO 04/000777 discloses a process for reacting di- and polyalkenes with dinitrogen monoxide to give the corresponding carbonyl compounds. The purification of dinitrogen monoxide is not mentioned in these documents.
It is likewise known that offgas streams comprising dinitrogen monoxide can be used for further reactions. Dinitrogen monoxide is obtained as an undesired by-product in various chemical processes, especially in oxidations with nitric acid and there very particularly in the oxidation of cyclohexanone and/or cyclohexanol to adipic acid. Other examples of processes in which dinitrogen monoxide is obtained as an undesired by-product are the oxidation of cyclododecanone and/or cyclododecanol with nitric acid to give dodecanedicarboxylic acid, the oxidation of acetaldehyde with nitric acid to glyoxal and the partial oxidation of NH3 to NO.
For instance, WO 2005/030690, WO 2005/030689 and WO 2004/096745 disclose processes for oxidizing olefins with dinitrogen monoxide, specifically the oxidation of cyclododecatriene, of cyclododecene and of cyclopentene. All three applications disclose that, in addition to other dinitrogen monoxide sources, it is also possible to use offgas streams which can be purified, for example, by distillative methods before they are used as oxidizing agents.
Both in the preparation of dinitrogen monoxide and in the use of offgas streams, N2O is obtained initially as a dilute gaseous mixture with other components. These components can be divided into those which have a disruptive effect for specific applications and those which behave inertly. For use as an oxidizing agent, gases having a disruptive effect include NOx or, for example, oxygen (O2). The term “NOx”, as understood in the context of the present invention, refers to all compounds NaOb where a is 1 or 2 and b is a number from 1 to 6, except N2O. Instead of the term “NOx”, the term “nitrogen oxides” is also used in the context of the present invention. Disruptive secondary components also include NH3 and organic acids.
For specific applications, it is necessary to purify the dinitrogen monoxide used before the reaction. For example, for the use of dinitrogen monoxide as an oxidizing agent, it is necessary to remove disruptive secondary components such as oxygen or nitrogen oxides NOx.
Processes for removing NOx are known in principle from the prior art. A review is given, for example, by M. Thiemann et. al in Ullmann's Encyclopedia, 6th Edition, 2000, Electronic Edition, Chapter “Nitric Acid, Nitrous Acid, and Nitrogen Oxides”, Section 1.4.2.3.
The application WO 00/73202 describes a method as to how NOx and O2 can be removed from an N2O-containing gas stream. The NOx is removed by catalytic reduction with NH3 and oxygen by catalytic reduction with hydrogen or other reducing agents. However, this method has the disadvantage that the product is contaminated with NH3. A high depletion of oxygen is possible only when a loss of N2O is accepted (of, for example, from 3 to 5% of the amount originally present).
For specific applications, it may be necessary also to remove the inert compounds, since they can slow the desired reaction with N2O by dilution. The term “inert gas”, as used in the context of the present invention, refers to a gas which behaves inertly with regard to the reaction of N2O with an olefin, i.e. reacts under the conditions of the reaction of olefins with N2O neither with the olefins nor with N2O. Inert gases include, for example, nitrogen, carbon dioxide, argon, methane, ethane and propane. However, the inert gases can lower the space-time yield, so that a depletion can likewise be advantageous. However, it may likewise be advantageous to obtain a gas mixture which still comprises inert gases, such as carbon dioxide, and then can be used directly in a further reaction.
DE 27 32 267 A1 discloses, for example, a process for purifying dinitrogen monoxide, wherein nitrogen oxide, nitrogen dioxide, carbon dioxide and water are initially removed and the gas mixture is subsequently liquefied by compression to from 40 to 300 bar and cooling to from 0 to −88° C. From this liquefied gas mixture, dinitrogen monoxide is then removed. Although this method achieves a purification and concentration of the N2O, it is economically unattractive owing to the required high pressure (60 bar), the low temperatures (−85° C.) and the associated high capital costs.
U.S. Pat. No. 4,177,645 discloses a process for removing dinitrogen monoxide from offgas streams which likewise comprises a prepurification and a low temperature distillation. The application EP 1 076 217 A1 likewise describes a method for removing low-boiling impurities from N2O by low temperature distillation.
U.S. Pat. Nos. 6,505,482, 6,370,911 and 6,387,161 also disclose processes for purifying dinitrogen monoxide, in which a low temperature distillation is in each case carried out in a special plant.
However, as a result of the high pressures and low temperatures, a low temperature distillation entails high apparatus demands, which make the purification of the dinitrogen monoxide with such a process inconvenient and costly. Particularly troublesome in this context is the fact that the melting point of N2O at standard pressure is only 3 K below the boiling point. It is therefore necessary to employ high pressures.
DE 20 40 219 discloses a preparation process for dinitrogen monoxide, wherein the dinitrogen monoxide obtained is concentrated and purified after the synthesis. According to DE 20 40 219, dinitrogen monoxide is prepared initially by oxidizing ammonia. The dinitrogen monoxide prepared is purified by separating the oxidized gases and concentrating by absorption under high pressure, which is followed by a desorption under reduced pressure. Secondary components are removed, for example, by treatment with an alkali solution in a wash tower. According to DE 20 40 219, water is used as the solvent for the absorption of the gas mixture.
It is possible with the process disclosed in DE 20 40 219 to separate the different nitrogen oxides, but the process entails the use of large amounts of solvent and/or high pressures for the absorption. At the same time, a further wash tower is needed for the process disclosed in DE 20 40 219 to remove further disruptive components.
WO 2006/032502 discloses a process for purifying a gas mixture comprising dinitrogen monoxide, which comprises at least one absorption of the gas mixture in an organic solvent and subsequent desorption of the gas mixture from the laden organic solvent, and also the adjustment of the content of nitrogen oxides NOx in the gas mixture to at most 0.5% by volume based on the total volume of the gas mixture. WO 2006/032502 also discloses that the process may comprise a plurality of absorption and desorption steps. WO 2006/032502 discloses only organic solvents as the absorption medium.
DE 10 2005 055588.5 relates to a process for purifying a gas mixture G-0 comprising dinitrogen monoxide, at least comprising the absorption of the gas mixture G-0 in an organic solvent, subsequent desorption of a gas mixture G-1 from the laden organic solvent, absorption of the gas mixture G-1 in water and subsequent desorption of a gas mixture G-2 from the laden water, and to the use of a purified gas mixture comprising dinitrogen monoxide obtainable by such a process as an oxidizing agent for olefins.
EP 06 125 807.5 relates to a process for purifying a gas mixture comprising dinitrogen monoxide, wherein absorption and desorption are effected in aqueous solvent mixtures at particular pH values.
However, with the known processes, small amounts of oxygen which remain in the dinitrogen monoxide can be removed only with difficulty. Specifically traces of oxygen may, however, lead to undesired by-products in subsequent reactions.
For example, in the oxidation of cyclopentene or cyclododecatriene, it has been observed that between 1 and 4 mol of the olefins used are consumed unproductively per mole of oxygen in the dinitrogen monoxide used, i.e. the presence of oxygen in the dinitrogen monoxide can lead to the formation of by-products, for example to the formation of deposits, which can then lead to a blockage of the rector.
Proceeding from this prior art, it was an object of the present invention to provide a process with which the content of oxygen in dinitrogen monoxide-containing streams can be reduced effectively and inexpensively. Dinitrogen monoxide purified in this way is required especially as an oxidizing agent.